1. Field of the Invention
This invention relates to acoustic vibration sensing apparatus having a light transmitting fiber.
2. Description of the Prior Art
Many sensors for acoustic vibrations and other fluid pressure effects are well known and have various deficiencies such as low sensitivity, high cost, limited maximum pressure, and errors due to temperature, pressure, and acceleration. The flexure or strain of an elastic disk subjected to an axial pressure difference is a well-known basis for acoustic and other pressure measurements, the amount of flexure typically being determined mechanically, piezoelectrically, or by changes in capacitance and resistance of elements associated with the disc. It is apparent that axial acceleration of the disk causes flexure thereof indistinguishable from flexure due to pressure differential, and all strain measuring devices have errors due to temperature caused changes in length or other dimensions, modulus of elasticity, index of refraction, and the like.
Interferometer measurements of strain can provide great resolution and, when carried out with an optical fiber, can provide a simple and rugged sensor which requires low power, is immune to many forms of interference, and is adapted to remote sensing of pressure variations and to high data rates. However, optical fibers are relatively insensitive per unit length when used for measurement of pressure variations and are subject to errors due to ambient pressure, tension from acceleration, and the like. Interferometers having an optical fiber leg are particularly subject to error due to variation of the length of the leg caused by temperature. Increase in leg length to provide greater sensitivity typically increases such errors proportionately. It is known to minimize these errors by a "push-pull" arrangement of a pair of interferometer optical fiber legs where a change in a measured variable shortens one leg and lengthens the other while both legs change length together with variations in temperature, pressure, and acceleration. However to be effective, this interferometric rejection of common mode errors requires that both legs be subject to exactly the same conditions.
Optical fiber interferometric measurements of pressure and fiber interferometric measurements of pressure and variations thereof may be carried out directly with such fibers of suitable construction. These measurements may also be carried out indirectly by arranging a resilient cylinder, which is wound with an optical fiber, for compression resulting from strain of a primary pressure or force measuring element so that lateral expansion of the cylinder lengthens the fiber. These fiber optic arrangements for pressure measurement are effective and adapted to push-pull operation, but are somewhat limited in sensitivity and existing designs have push-pull optical fiber legs separated spatially and thermally to an undesirable extent.